Mass spectrometry



Patented oct. 7, 1952 *t 2,613,323

-UN1TEDs-'1'ATES MTENT oFFlcr. j

MASS SPECTROMETRY George D. Perkins, Pasadena,V Calif., assignor to Consolidated Engineering lCorporation, 4Pasadexia. Calif.. a corporation ofCalifornia 1 ApplicatioaNovembfls,1949,'seria1N0.127,447

` 16 clair-11a (Crm-#419) `This invention relates to mass spectrometr and to mass spectrometers. y .More` specically, the invention is directed to improvements in the method of focussing ion beams and to apparatus for carrying out the improved method. e i

A- mass spectrometer isman analytical instrument `for sorting and `measuring ions. Ordinarily. it includes van ionization chamber in which molecules of the sample to be analyzed are bombarded with a stream of electrons to-convert them into ions.` The ions formed are expelledirom the ionization chamber by onev or more repeller electrodes and are propelled by so-called propelling orfaccelerating electrodesfinto and through an analyzer. chamber. e The analyzer chamber frequently takes the form of acurvilinear enclosure, say a tube. During passage through the chamber, the heterogeneousv beam of ions is subjected to a. transverse electricalormagnetic field or both,`to separate it .into separatefdiverging ion beams, each of which vis composed of ions ofl a given ,mass-to-charge ratio whichdiiers from the mass-tofcharge ratio of the ions forming the other beams. The diverging beams may be successively focussed through anexit lor resolving slitin a focussing electrode ".:ontora collector electrode. Alternatively, a single one of the ion beams finay be continuously focussed on the resolving slit to the exclusion of the other beams. In either` event, 4thefocussing Vof the ionv beam or beams is achieved by adjustment of the potential applied to the accelerating electrodes. The current produced by ion discharge at the col-v lector .electrode is indicative of the amount of ions` in the particular beam and hence, constitutes'a measure' of thepartial pressure `of the molecules (from which the ions were derived) inthe 'sample being analyzed. e

As indicated above, it may beexpedientoto focus the mass spectrometer on a single mass peak or step-.wisevon two or more mass peaks rather than sweepingV the diverging ion `beams successively across the resolving slit. Such a xed :focust'` instrument is used for `continuous monitoring where a continuous concentration of a single constituent in a sample stream is sought, inleak detection where the presence of an added contaminant is detected las it appears `inthe atmosphere surrounding a vessel under investigation, and generally under any circumstances e infwhich a single component or a pre-selected group of components is to be measured. i

For accurateanalysis with afixed focus mass spectrometer,it is important that the particular ion` `beam under investigation be at all times centered on the resolving slit. Any appreciable deection of the subject beam from a centered position will affect the apparent intensity `of the beam and hence, result in erroneous analysis. Since the factorswhich'governthe positionof the beam are the transverse magnetic eld and the accelerating` voltage,` deviation of either of e these variables from a pre-selected nxed value will result in displacement of the beamfrom a centered position.

It has been proposed tostabilize` the accelerating voltage against deviations from a preselected value by means of l a voltage-dropping network wherein a fraction of the accelerating voltage is compared with the voltage output of `a standard cell. `Any variation in proportionality between the fraction and the standard voltage is ,balancedrautomatically by a servo-motor or other means acting onthevoltage source supplying the acceleratingl electrodes. L However, the .accuracyof` sucha system `depends upon `a `resistance divider network, the long-term.stability'of` which is questionable. ltlloreover, the possibility of'rloss of focus due to temperature sensitivity, variations in vpower supplyto an electromagnet, oraging of afpermanent-,magnet is `innoway accountedpior in such a voltage-droppingnetwork. e e

I have developed a method for holding a given ion beam vc entered'onthe resolving slit-which automatically compensates for fluctuations in the accelerating voltage and variations inthe strength of `the magnetic eld. 'I'he invention contemplates in a method cimass spectrometry which comprises-ionizing a sample to be analyzed,

`propellingthe ions by means of-aD'. C. a ccel erating potential into an analyzer region, subjecting the ions inthe analyzer region t0 a trans-` verse magnetic eld to separate theminto indi-` vidual ion beams; focussing one of said ionbeams through a resolving slit in a terminal electrode,

discharging the beam passing through the resolv-v ing slit, and sensing the current developedv by discharge of the beam,` the improvement com# prising passing the ion beam through `a sensing slit prior to discharging it, superimposingpn the D. Craccelerating potential an A. C. component to cause the'ion beam `to `oscillate transversely with respect the sensing slit, `detectingnncenl- 'Gered oscillation or thebeamand adjusting the accelerating potential to restore centeredl oscilla- 1on. v

, Theinvention also includes one form of apparatus for carrying.` out this method, which comprises in a mass'spectrometer having an' analyzer tredeL plie' avlftage supply 'circuit `I4,-"including"a D. C.

' voltage source I 5 connected infserie's with a slide- -wire resistor VI 6;" Resistor f6 is1 tapped byl'slider I I which is connected lthrough a second resistor I8 chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connectedito 'develop an accelerating potential on the accelerating electrodes, means for establishing a transverse magnetic eld across the analyzerv chamber, a collector electrode, and a terminal electrode r:having a resolving slit through, which an ionpbeani is -f ocussed on thelcoll'ector'elec--- trode, the combination' comprising'a source of A. C. voltage connected to impress an A. C. com-..

aciasza to the vaccelerating electrodes responsive to un'- centered oscillation of the ion beam at the sensing slit to restore the beam to a centered positiom.

acrdance with-- th'ei'nvention andy .Y a; sensing slittherein, andgmeansY operative to vary the voltage applied from 'the D..C.Isource 4 is electrically connected to D. C. amplifier 34 which amplies the discharge signal and feeds it to a sensing means 36 which may be a recorder or other sensing device. To this point the operation of the mass spectrometer of Fig. 1 is typi cal of so-called fixed-focus instruments. v

In` accordance with thepresent invention, a

source y38. of A. C. voltage-is coupled through a sv'vitch'38Aa1`1d'a transformer 39 to the voltage supply circuit I4 to superimpose on the D. C. volt- Vage applied to the accelerating electrodes a comparatively small A. component of a frequency w. y

Asensor electrode 40, is interposed between thek ,focussing electrode-30 and collector electrode 32. A s'ensingslit 42 is provided in the sensing electrode whichis narrower than the resolving slit y Y y. 3| but wider than the ion beam by an amount The invention will .be more clearly understoodv speetivelv the resultsv in-term's of theoutput sigbeam 'isdecenteredt 'the rightf the slit. The mass` spectrometer shown diagrammaticauyfin Figi incu1de a'pai'r of `acceieratingelectrodes", Iliff' II!- and*a-'re'peller 'electrode I2. Ions formed inthe' {ionizationf region between fthe rep'elle'r' Aelectrodevv and f' 'the' rst' accelerating elecv 'are expelledgf5rom vthis re'g'lon through a slit 4 the 'accelerating electrode', by application f a' positive potential to,y the "repeller electrodefv1 f 'f The.z lexpelled' ions are "accelerated "be- 1 tween-the electrodes-Ij'andfl II and are' discharged 'from'i-a slit-'II-Afin thes'eond electrode by means potential `impresse`d Y between the: /twof ac- 'epellingand accelerating`I voltages alley vv2li"- the.. repeu and accelerating electrodes tq'vfthe'fposinve side cfftn D., C: source 15. VThe 'res trieroffremcterrorntnslider*l r1.i A ieadjzz con- 'r'i'ectsthe'repellerfel-ectrodeto'resistor II!V on the positvefsideof ground 20l and leads 23,V 24,v re- 'spectlvely`,' connect the two accelerating elecltrodegl-II),1II to spaced points onl theresistor I8 on thenegativeside 'ofgr'ound '.2'0. This arrange-r mentrepresentsl a conventional voltage supply circuitP-iforwaf'.fiXed-focusQmassspectrometer Vwherein any change irithe"voltage` delivered from thesourceior applied acrossresistor I 8 results in y proportional"changeinithe voltages applied tol the `two:accelerating e1ectrodes.rf'

ratio maybe focussed on a collector electrode 32' 1 disposed :behind: thee-terminal electrode'.` vIons passing through'the, resolving slit 3 I' ldischarge on the collecter-electrode The: collector electrode equal to a safe tolerance for departure of the ion beam from a centered position.A Electrode 40 i'sionnected to anA. C. ampliiier 44.1 A ,motor 46 isf mechanically' coupled to thefDfCf voltage ply circuit,v as for example; to 'theslider LI1 'so that-actuationof the motor controls'the I tion of slider I1', depending`;upon'th'e direction of motor rotation. One cil 45A"oftheinotor' vis connccted to tlfieA.y C. source 38 andv thev otherfcoil '46B' of'the motor is connected to the A. C. amplineru. f f i InA operation, the ion 'beam is voscillateclfby superi-mposing'fan A.- C/[voltageon the accelerating electrodes, thel magnitude of. the ysuperimposed A. Ca voltage beingsuch" as tojjdevelop beam oscillation having anampl'itude Vless than the width of lthesensi'rig' slit42. "Under such circumstances,`o`scillation of the beam will have'no effect on'theDfC. componentof the*l signal de veloped at Vtl'ie'V collectorelectrode.l `However, if

. either of the vvfacto'rsf,I namely; accelerating :potential orma'gneticfiield strength, varies'suiciently so that ydeviatio'mof the beamjroma centered position causesf it to 'strike one sidefor the other o f `the sensingislit during oscillation, a" signal will be produced infthesensing slit and vintro- V duced to the no. ampiiner 44; i p

The'voperationyof kthesysternof Fig. 1 is shown diagrammatically in FigsL'ZtZA `and 2B lunder con-j ditions whereinfthe ionbeam'fis centered, dis'- placed to the left of center, and displacedto the right of center, respectifvely,v allV with respect to the sensing vslitl 42;l In Fig. 2, the positionof the ion beam 26 with respect to the sensing slit A42, when the beam is oscillating about a centered position Aat frequency Iq isfshown.' `Hence, the zeroposition ofthe beam, Ias-jdekaicted at the eX- treme left of Fig. 2, iscentered on `the sensingslit andwith an amplitudeless thanthe ,Width of the slit, the beam remainsl in 'the slit during oscillation and no signal is` developed. In Fig. 2A, with the beam zero position to the left of center. a pulse` is formed each time the beam falls outside the sensing sli t 'Such condition prevailsin the quarter cycle immediately to the right of the zeroiposition and again'i'n 'the quarter cyclev following thecoinpletion of a cycle, etc'. Pulses., SUA'are' developed at the'sensor electrode giving an"'A.'4C. curvelSI upon Fourier'analys'esj having-a fundamental' frequency w. These'pulses are'ampled inthe A. C. amplier 44A and are 'fed'tofoil Y electrodes.

arranged transversely with respect tothe path of l ion travel will induce the same type of oscillation form and frequencyy but of exactly oppOsite phase from the pulses formed when thebeam is oi center to the left. When the pulse, shown in .Fig. 2B, is amplified and appliediacross coil 46B. ythe motor 45 will be drivenin direction opposite to' that resultingfroma pulse of Athe'typefshovvn in Fig. 2A. i

VIn this manner, if the beam focusdeviatesirom either side 4of center any appreciablearnount,

`motor 46 will be immediately ,energized rtoladjust' the position of slider VIl in a compensatory direction so that oenteredfocuswill be regained.

AThe method and apparatustdescribed h as the important advantage Vof correcting for all eiects contributing to undesired loss of focus of` the fpre-selected ionbeain, A highdegree of sensitivity is. not necessaryyin thefA.. C. amplifier since this potential in the manner illustrated in con'- junction with the accelerating electrodes. :A's still l a thirdy alternative, the magnetic field, Whether developed byy an electromagnet or permanent magnet,` maybe varied responsive tov-the lignal developed by Vany*citi-centered oscillating eam.

The method of the invention, therefore, *in- `volves Aoscillating the ion beam in any desired manner and adjustingfa beam focussing neld fvvhetherit be a lateral or transverse electric the device can be operated to center on a stronger t peak than the particular one `under investigation,

if desired. Thus, if the particular peak oiinterest is Weak, a stepper switch. can be includedV in the D. C. supply circuit `I4 to periodically alter the` accelerating voltage to focus a stronger peak on the resolving slit. 'Under` suchL circumstances,

the A. C. voltage component can be appliedto .the` accelerating 'electrode .only ,when this "stronger` peak .is in focus. Conveniently` this selectivity is obtained by. gauging such a stepper switch to the switch 38A governing the applicationof an A. C. voltage from the source 38.

i field`fv or atransveiseniagnetic field responsive' to 'af characteristic signal-developed by the oscillating lbeam Vwhen the mean `posi tion' 'of the beam 'is not properlycentered o n the resolvingslit:

l.` In massYspectri'irnetry involving ionizing a sample to lbev analyzed, propelling theions"v by meansjof a D;",C`.`Vacceleratingjpotential intdan analyzer' region, subjectingl.: the ions "in "the analyzer region to a 'transverse magnetic eld to separate them into individual'-ion beams. focussingone of said ion beams'through a resolv"- ing slit in a terminal electrode, discharging the beam passing through the resolving ,slitfand sensing the current developed 'by' discharge.' of

the beam, the improvement `.comprising passing ,the ion beam throughasensingfslitlafter passage Constancy of amplification in the A. C. `amplit fier is also not a problernprovided only thatthe gain remains large `enough to Vvdrive the motor.

The system is not adjustedresponsive tojth'e `magnitude of the A. C. amplierjoutputso that variation in gain `Within the range sufcient to 4operate the motor can be" tolerated@` Furthermore, motors can be obtained 'commercially b which automatically f discriminate `ibetvveen "noise land extraneous frequencies Which, therefore,V do

not interfere with the operation of the apparatus .so long as they are not present in sufficient amounts to jamb the motorsby `over damping.

The application of theV invention is simpler,

`less expensive, and morecertain than any means Vheretofore proposed for holding ,a mass., spec.-

trometer on the top of a pre-.set peak or a` Vgroup of peaks byattempting to achieve a highdegree of long-term stability in the magnet 'andin the A power supply.

Anhougn the inventin has `been described .with particular reference to a continuous monitor instrument in which a single mass peak is' of interest, it `is `not so limited.A In any operation v wherein the mass peaks are focussed step-Wise lon theresolving slit, the present invention is of value.

Furthermore, application of the invention is not limited to the particular apparatus shovvn.

`It is entirely practical to produce the necessary oscillation of the ion beambyl means other than impression of an A.`C. voltage on the accelerating For example, j auxiliary electrodes when energized with an A. C. component. Alternatively, the magneticiieldmay be varied in a rhythmic manner for the same purpose.` AAt the same time, means other` than .adjustmentof the accelerating potential may be `employedto maintain or restore centered focus independent of the particular expedient employedto oscillate `the ion beam. Again, transversely, arranged Yelectrodes may serve this focussing function ,by Yimpressing a D. C. potential thereon and varying beam to oscillate. transversely With'respect:

throughthe resolving slit and priortodischargfing it, superimposing o'n'the' D'.` C`.` acceleratirng potential an A. jC.`cor'nponent to causevtheion fthe sensing slit,l detecting uncentered oscillation of Athe beam and adjusting the acceleratingpotenn `tial to` restore "centered v,oscillat'.io'n. s

Q.2. In mass spectrometry'involving ionizing a sampleto be analyzed, `propelling the ionsfby meanspf a D. C. kaccelerating potential' into jan analyzer region. subjecting. the; ions` in 3` the analyzer region to a transverse magnetic neld Ato separate` them into, individual I ion beams, .focussing one of said ionbeams through a resolvingslit in a terminal electrode,'dischargingjthe beam passingV through the Vresolving slit and sensing .the current. developed byA discharg'eiof lthe beam, theimprovement comprising passing the ionbeam through a sensing slit aftenpas'- sage through the resolving slit and vprior to charging. it, superimposing on thegDl." C. accelerating. 4potential ani A.' component lto cause the ion beamto oscillate transversely Withl re.-

spectto the sensing s'lit and with afn'ramplitude les'sthanthewidth ofthe sensing slit, detecting uncentered, oscillation of the beam 'and adjusting the accelerating potential to 'restore'centered oscillation..

3. In mass spectrometry -involvingionizingfa `sampleto be analyzed, propelling the ionsby means of a D. C. accelerating potential into an analyzer region, subjecting the `ions in` ythe analyzer region to a`v transverse' lmagneticl field to separate' them into individual ion beams, focussing'one of said ion beams through a resolv'- ing slit in wterminal electrode, discharging the beam passingfthrough the resolvingslit 'and sensingthecurrent developed by discharge ofthe "beam, 4the improvement comprising passing the ion beam through a'sensing slit after' passage through the resolving slit andprior to dischargingthebeam, superimposing on the DfC'faccelerating'potential any A. C. componentv to cause the ion beam to oscillate transversely 'Withf'fre'lspectto the sensing slit and with'` an `ar'nplitu'cle lessthanthe Width of the sensingslit, detecting uncentered oscillationv of the bea-m and adjusting Jseparate them into V.affissione' l l .u .....c.

ion beam .through asensing slit in asensing eleca.'

trode between the terminal and collector `elec- ;trodes, periodically .superimposing on the D.` C..ac rceleratingrpotential an A. C. component to cause the. Aion lbeam to oscillate. transverselyl with .re- :spectto .the sensingslitV and with an amplitude less than .the Width of the vslit whereby theoscillating beam .generates an A. C. signal at the sensingelectrode when itis not centered on the l.sensingslih and ladjusting the laccelerating po- ,tential vresponsive to l.said A. C. :signal to vrestore centered oscillation.

`5. "In .mass .spectrometry ,involving ionizinga ,sample .to'bef analyzed, V'propelling'the ions byV l'rieelsl 'Offay DL' C. raccelerating potential into an .analy'zer,1"egifo`r'i',v subjecting theions in the fanaly'zenregion" to a `.transverse .magnetic field to ,slepa'ratel "themy into individual. vion beams, focussing one :of said ionv beams through a v resolving .slit in'ja' terminal electrode, discharging the beam passingy throughfthe resolving lslit and :sensing vthe currentdeveloped 'by discharge of the ,'beai'n,v the 4illlprovement'comprising passing the ionibeamthrougha sensing slit in asensing electrode ,between the terminal andcollector `electrodes', continuously superimposing on the D. C. accclerati'ng .potential 'an A. C. componentjto cause .theion beam to oscillate transversely `with respect to thcsensing sur and with an amplitude .less thanthe' width, of the slit whereby 'the os- .'cillating beam generatesanA. C. signal at the sensing 'electrode when itis vnot centered on the lsensing slit, and adjusting'the accelerating potenftial'responsive to r'said' A. C. signal to restore centered oscillation. l

n (i. 'In a massfspectrome'ter having an analyzer f chambenionization chamber, accelerating electrodes, 'a source of D. C. lvoltage connected to develop'an accelerating potential on, the acceleratiing' electrodes, vmeans Afor' establishing a trans- 'yeseflmagnetic field across the analyzer chamber, la collector electrode, and a terminal .electrode having a resolving slit through which an '.lonfbea'mis focussedon the kcollector electrode, the combination comprising a source of A. C. .voltage connected to impressan A. C. component fongthe accelerating electrodes tocause oscillationof the ion beam, a sensor electrode vdisposed .intermediate the terminal electrode and the col- .,lec'tor electrode and having a sensing vslit therein fand meansoperative to vary the voltage applied from the D. lC. source to the accelerating electrodes Iresponsive to uncentered voscillation of .the ion beam` at the sensing slit to .restore .the beam to. acentered position.

;. 7. ,In la mass spectrometerhaving ,an analyzer 7 ,chambenfionization chamber, accelerating electrodes, a source of D. C. voltage connected tode.-

.velQpan `z'tcceleratin-g potential on ,the accelerat-A iinglelectrodes', .means for establishing a trans- ;mag'netic ileld .across .the analyzer cham- -berpa .collector electrode, and .aterminal ...electrede having .a resolving. -Slit thrughwhichen fion-beam .isufocussed on the ...collector 4.electrode,

the combination .comprisinga source of .A.` voltage connectedito,impress.an;A. C.. component on the accelerating .electrodes to cause `.o'sc.`i1lati .o,n of the ionA beam, a sensor electrode disposedintermediate d the. terminal ,electrode and' .the collectorelectrodefand having a sensing slit therein, means fordevelopingfanA. C. signal-.respon- .sive,-toluncetercd'oscillation of the beam with respect the sensing gs'lit,y .andmeans loperative Ito vary the/voltage applied' from the D., C.."soi1`r'c'e .tolthe accelerating electrodes responsive `to .said

A'. C. signal-to restore the. beam to a centered position. wf;

8. In a .mass spectrometer having an analyzer chamber, ionization chamber,vv accelerating' electrodes, a source of D. C. `voltage connectedf'to develop an accelerating potential kon the accelerating electrodes, means for establishing a transverse magnetic eldacross the analyzer chamber, a collector, electrode, yand a terminalelectrodefhaving a ,resolving slit through'which an .ion beam is focussed on the collector'electrode, the combination comprising alsensormeleotrode l L,disposed intermediate the terminal electrodea'n'd the collector electrode and having a sensingl slit therein. a. source of A. C. voltage connected Lto impress .an Aj. C. component oncthe .accelerating electrodes to cause oscillation of fthe `ion beam,

the sourcev of A. .C.voltage being adapted to produce an A. C. componentvat the accelerating elec'-A "trodes of suchmagnitude as to cause beam oscillation Jof an amplitude less than .the Width of the .sensing slit, means v'for developing'y an A. C.' signal responsive to .uncentered oscillation y of y 'the beam ,atxthe sensing slit, and means operative tofvary thefvoltage applied from thel). C. ,s o u`rce lto the accelerating electrodes respor'isive'v tov said A." C. lsignalto vrestl'lre the' beam ,to a centered posi'- tion.

9; In a mass spectrometer having an lanalyzer chamber, ionization chamber, acceleratingelecf', trodes, a source of D. C. voltage connected to'develop `an accelerating potential' on the, accelerate ving electrodes, means for establishing a transverse magnetic eld across the analyzer. chamber,` `,a lcollector electrode," anda terminal electrode having a resolving slit through which an ion beam is focussed on the collector electrode, the combination vcomprising a source of A. C. voltage connected to impress anA.v` C. component `on the 4accelerating electrodes to 'causejoscilla- .tion o! .the ionbeam with respect to the resolving slit, a sensor electrode disposed intermediate tne .singlone 4of said ,ion beams through are'solving slit and sensing the current developed by discharge of the beam, the improvement comprising ,passing the ion beam through a sensingjslit vafter passage through 'the .resolving slit and'priort'o .discharging it, 'superimpo's'ing 'on the' D. vCaccel- .erating'potential an A. C.' `component, 4to ,cause the ion beam to oscillate transversely with respect tothe sensing slit, detecting uncentered oscillation of the beam and adjusting the accelerating potential to restore centered oscillation.

11. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of a D. C. accelerating potential into an analyzer region, subjecting the ions in the analyzer region to a transverse magnetic eld to separate them into individual ion beams, focussing one of said ion beams through a resolving slit in a terminal electrode, discharging the beam passing through the resolving slit and sensing the current developed by discharge of the beam, the improvement comprising passing the ion beam through a sensing slit after passage through the resolving slit and prior to discharging it, superimposing on the D. C. accelerating potential an A. C. component to cause the ion beam to oscillate transversely with respect to the sensing slit and with an amplitude less than the width of the sensing slit, detecting uncentered oscillation of the beam and adjusting the accelerating potential to restore centered oscillation.

12. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by n less than the Width of the slit whereby the oscilmeans 0f a D. C. accelerating potential into an analyzer region, subjecting the ions in the analyzer region to a transverse magnetic eld to separate them into individual ion beams, focussing one of said ion beams through a resolving slit in a terminal electrode, discharging the beam passing through the resolving slit and sensing the current developed by discharge of the beam,

the improvement comprising passing the ion beam through a sensing slit after passage through the resolving slit alHd prior to discharging the beamfz.

superimposing on the D. C. accelerating potential an A. C. component to cause the ion beam to oscillate transversely with respect to the sensing slit and with an amplitude less than the width of the sensing slit, detecting uncentered oscillation of the beam and adjusting the accelerating potential responsive to the detected uncentered oscillation to restore centered oscillation.

13. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of a D. C. accelerating potential into an,

analyzer region, subjecting the ions in the analyzer region to a transverse magnetic field to separate them into individual ion beams, focussing one of said ion beams through a resolving .1

slit in a terminal electrode, discharging the beam passing through the resolving slit and sensing lating beam generates an A. C. signal at the sensing electrode when it is not centered on the sensing slit, and adjusting the accelerating potential responsive to said A. C. signal to restore centered oscillation.

14. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of a D. C. accelerating potential :into an analyzer region, subjecting the ions in the analyzer region tc a transverse magnetic iield to separate them into individual ion beams, focussing one of said ion beams through a resolving slit in a terminal electrode, discharging the beam passing through the resolving slit and sensing the current developed by discharge of the beam. the improvement comprising passing the ion beam through a sensing slit in a sensing electrode located between the terminal and collector electrodes, continuously superimposing on the D. C. accelerating potential an A. C. component to cause the ion beam to oscillate transversely with respect to the sensing slit and with an amplitude less than the width of the slit whereby the oscillating beam generates an A. C. signal at the sensing electrode when it is not centered on the sensing slit, and adjusting the accelerating potential responsive to said A. C. signal to restore centered oscillation.

l5. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of a D. C. accelerating potential into an analyzer region, subjecting the ions in the analyzer region to a transverse magnetic eld to separate them into individual ion beams, focussing one of said beams through a resolving slit, discharging the beam passing through the slit and sensing the current developed by discharge of the beam, the improvement comprising passing said ion beam through a sensing slit after it has passed through the resolving slit, causing the beam to oscillate transversely with respect to the sensing slit, detecting uncentered oscillation of the beam and altering the beam focus to restore centered oscillation.

16. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes for propelling ions from the ionization chamber through the analyzer chamber, means for establishing a transverse magnetic field across the analyzer chamber, and a collector electrode separated from the analyzer chamber by an auxiliary electrode having a resolving slit therein, the combination comprising a sensor electrode having a sensing slit therein and disposed between the auxiliary and collector electrodes, means for oscillating an ion beam with respect to said sensing slit| and means operable responsive to uncentered oscillation of the beam to vary the beam focus to restore centered oscillation of the beam.

GEORGE D. PERKINS.

No references cited. 

